The present patent application was filed during the pendency of Applicant""s earlier application (Ser. No. 08/381,156), which was filed on Jan. 31, 1995. That application (Ser. No. 08/381,156) was filed during the pendency of Applicant""s earlier application (Ser. No. 08/034,694), which was filed on Mar. 19, 1993. That application (Ser. No. 08/034,694) was filed during the pendency of Applicant""s earlier application (Ser. No. 07/862,313), which was filed on Apr. 2, 1992. That application (Ser. No. 07/862,313) was filed during the pendency of Applicant""s earlier application (Ser. No. 07/521,399), which was filed on May 10, 1990. That application (Ser. No. 07/521,399) was filed during the pendency of Applicant""s earlier application (Ser. No. 07/396,916), which was filed on Aug. 22, 1989. The disclosures of these prior applications are incorporated herein by reference.
Application Ser. No. 07/521,399 matured into U.S. Pat. No. 5,128,782, which issued on Jul. 7, 1992, and application Ser. No. 08/034,694 matured into U.S. Pat. No. 5,416,496, which issued on May 16, 1995. Application Ser. No. 07/396,916 and application Ser. No. 07/862,313 have been abandoned.
Although at the time of filing the present application, Applicant does not claim the benefit under 35 U.S.C. xc2xa7 120 of any of the chain of co-pending applications identified above, Applicant reserves the right to claim such benefit if, at any time during the pendency of the present application at the Patent and Trademark Office or thereafter, prior art turns up which makes such a claim for the benefit of an earlier prior date desirable.
The present invention is directed to a technique for using a spatial light modulator to display an image and, more particularly, to a technique for using a spatial light modulator having stable pixels to display a color image having gray scale gradations.
A digital micromirror device is a spatial light modulator which employs an array of tiny mirrors, or micromirrors, whose positions can be electrically controlled in order to display an image. This technology has been developed extensively by Larry J. Hornbeck and his colleagues at Texas Instruments, Inc. of Dallas, Tex., and is described by them in a sequence of patents going back more than a decade. These developmental efforts have culminated in a digital micromirror device which includes an array of memory cells and a corresponding array of pivotable micromirrors whose positions are electrostatically adjusted by the contents of the memory cells. As is perhaps best described in U.S. Pat. No. 5,096,279 to Hornbeck et al., the array of pivotable micromirrors that cooperates with the memory cells can be made using integrated circuit fabrication techniques.
As described in the above-identified patent, in U.S. Pat. No. 5,280,277 to Hornbeck, and in an article entitled xe2x80x9cMirrors on a Chipxe2x80x9d that was published in the November 1993 issue of IEEE Spectrum at pages 27-31 by Jack M. Younse, a negative biasing voltage is selectively applied to the micromirrors and to landing electrodes fabricated beneath them in order to obtain bi-stable operation of the micromirrors and simultaneous updating of the entire array of micromirrors. Sometimes, the micromirrors get stuck. It is known that this problem can be solved by subjecting the micromirrors to resonant reset pulses which electrostatically dislodge any stuck micromirrors.
It is also known to make a color display using a single digital micromirror device by sequentially exposing it to red, green, and blue light impinging from a single direction. A white lamp and a color wheel can be employed for this purpose. Gray scale gradations can be achieved by exposing a digital micromirror device to light for different time intervals that are determined in accordance with the rank of bits of video information displayed on the digital micromirror device, as disclosed in U.S. Pat. No. 5,452,024. Furthermore, the light shining on the digital micromirror device may be generated by a lamp that is driven by an amplitude modulated driving waveform, as disclosed in U.S. Pat. No. 5,706,061.
Advances have also been made in display devices which employ other types of spatial light modulators. For example, U.S. Pat. No. 5,122,791 to David J. Gibbons et al discloses a ferroelectric liquid crystal display panel (which has bi-stable pixels with a fast response time) as the spatial light modulator. It is selectively back lit by red, green, and blue fluorescent tubes, and the intensity or duration of the back-lighting is controlled on the basis of the rank of the bits that are being displayed on the LCD panel.
Applicant""s Pat. No. 5,416,496 also employs a ferroelectric LCD that is back-lit with colored lights. The colored light may be generated in flashes whose intensity is controlled on the basis of the rank of the video information bits that are being displayed. Alternatively, instead of flashes of light, the LCD panel may be illuminated by light that is generated steadily, and whose intensity is determined by the rank of the bits that are being displayed. In the latter alternative, the pixels of the panel are turned on in accordance with the video information on a row-by-row basis, and are subsequently turned off in accordance with the same video information, again on a row-by-row basis. As a result, each pixel that is turned on and then turned off receives the same amount of light regardless of its row, so the LLD can be addressed row-by-row with video information while the LCD is being illuminated.
An object of the invention is to provide a display apparatus which employs an addressable spatial light modulator that is illuminated by a lighting unit whose light output varies in intensity in accordance with the bit rank of video information that is being used to address to the spatial light modulator, with the light output of the lighting unit being monitored in order to determine when to change what is displayed on the spatial light modulator. The video information may be fed to the spatial light modulator on a frame-to-frame basis for each color, or on a row-by-row basis for each color. If the video information is fed to the spatial light modulator on a row-by-row basis, the amount of light received by different rows can be equalized, during display of a particular bit rank of video information for a particular color, by turning the pixels on row-by-row in accordance with the same video information.
Another object of the invention is to provide a display apparatus which employs a spatial light modulator that is illuminated by a lamp unit having a plurality of lamps, with the light intensity being adjusted by turning at least one of the lamps on and off.
Another object is to provide a spatial light modulator that is illuminated by a lamp unit having a single lamp that is driven at different intensities, depending on the bit rank that is being displayed. Instead of a single lamp, a plurality of lamps that are driven in unison may be used. For example, a plurality of lamps may be connected in parallel to supply more light than could be delivered by a single lamp.
A further object of the invention is to provide a spatial light modulator that is illuminated by a lamp unit which emits light with an intensity that is constant, with the intensity being controlled before the light impinges on the spatial light modulator (or after impingement on the spatial light modulator, if preferred) by passing the light through at least one attenuator. The at least one attenuator may be a plurality of rotating attenuators, possibly combined with a color wheel. Alternatively, the at least one attenuator may be a liquid crystal panel having rows that are selectively turned on in accordance with the desired light intensity, or a liquid crystal cell which is pulse-width modulated in accordance with the desired intensity.
A further object of the invention is to provide novel techniques for illuminating a spatial light modulator through a rotating color wheel. If the color wheel is rotated more than one revolution during display of a frame of video information, different bit ranks of the video information can be allocated to different revolutions. Furthermore, the most significant bits can be partially displayed during one revolution and subsequently completed during one or more additional revolutions.
A still further object of the invention is to integrate the light emitted by a lighting unit whose intensity is changed through a plurality of levels in order to control the duration of buffer periods which accommodate relatively slow changes in the light intensity or erratic light output during transitions from one level to another, the buffer periods being periods when the data displayed on the spatial light modulator is such that all of the pixels of the spatial light modulator are turned off. The buffer periods may have durations that are controlled by monitoring the light generated by the lighting unit. The buffer periods may also have fixed durations, corresponding in duration to the time needed for a color wheel to rotate completely through one or more colored sectors or through one or more complete revolutions.
In accordance with one aspect of the invention, a method for using a spatial light modulator can be conducted by displaying data on the spatial light modulator, shining light on the spatial light modulator, integrating the light, and changing the data displayed on the spatial light modulator when the integrated light reaches a predetermined value. The method may further include changing the intensity of the light shined on the spatial light modulator, either by using a lighting unit having a plurality of lamps and turning at least one of the lamps on and off, or by using a lighting unit having a single lamp that is driven at different energy levels during a sequence of time periods. This latter alternative may be modified by driving a plurality of lamps, in unison, at different energy levels during the sequence of time periods.
A color wheel may be used to color the light, preferably (but not necessarily) before it impinges on the spatial light modulator. The color wheel may be rotated at a rate faster than the frame repetition rate. This can lead to several advantages. One is that some of the bit ranks for all three primary colors can be displayed during one revolution of the color wheel, and other bit ranks can be displayed during one or more subsequent revolutions. Buffer periods can be used to adjust the amount of illumination received by the spatial light modulator in accordance with the bit ranks. Another advantage is that the display of the most significant bits for a frame may be spread over two, and possibly more, revolutions of the color wheel. This means that the total amount of light of a particular color that impinges on the spatial light modulator is not limited by the product of the light intensity and the time needed for the color wheel to rotate through a single colored sector. For example, the spatial light modulator may be illuminated with red light during display of the most significant bits of the red component of an image for a period corresponding to the rotation of the color wheel through an angle of 200xc2x0, with half of this angle plus a buffer period occurring during one revolution, and the other half plus another buffer period occurring during another revolution. Illumination for the green and blue components can, of course, also be conducted in this manner. A further advantage is that buffer periods, when all of the pixels are off, may be inserted during rotation of the color wheel through one or more colored sectors or through one or more complete rotations to absorb slow or turbulent transitions from one light-intensity level to another.
According to a related aspect of the invention, a method for using a spatial light modulator can be conducted by displaying data on the spatial light modulator, shining light on the spatial light modulator, coloring the light with a color wheel (preferably before the light impinges on the spatial light modulator, but possibly after impingement of the light instead), and rotating the color wheel faster than the frame repetition rate. The method may further include integrating the light and changing at least some of the data displayed on the spatial light modulator when the integrated light reaches a predetermined value. The most significant bits for all three primary colors may be displayed during two or more revolutions of the color wheel, and different bit ranks for all three primary colors may be displayed during different revolutions. Furthermore, the intensity of the light shined on the spatial light modulator may be changed as the color wheel is rotated.